Lithium-induced supramolecular hydrogel

Article abstract of DOI:10.1039/c1cc10532a

We describe a novel anisotropic supramolecular gel made of cyclodextrin-dye, in which physical gelation is completed by lithium salt. Rheological experiment reveals the elastic behaviors of the hydrogel, and high ionic conductivity represents a good mobil

Full text of DOI:10.1039/c1cc10532a

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COMMUNICATION
Lithium-induced supramolecular hydrogelw
Jong S. Park,*a Sooyeon Jeong,a Dong Wook Chang,b Jae Pil Kim,c Ketack Kim,d
Eun-Kyoung Parke and Ki-Won Songe
Received 27th January 2011, Accepted 3rd March 2011
DOI: 10.1039/c1cc10532a
We describe a novel anisotropic supramolecular gel made of
cyclodextrin–dye, in which physical gelation is completed by
lithium salt. Rheological experiment reveals the elastic behaviors
of the hydrogel, and high ionic conductivity represents a good
mobility of ions inside the gel matrix.
In spite of the fascinating results to date, the demand for the
development of novel supramolecular hydrogels has increased
steadily. Furthermore, only a few studies have been done on
CD-based supramolecular gels triggered by a specific metal
ion. As far as we know, supramolecular gels induced with
lithium ions are quite rare.4 In this paper, therefore, we
describe a novel anisotropic supramolecular gel of CD–azo
dye, in which physical gelation is completed by a specific metal
ion, that is, lithium salt. Therein, we examine whether the
incorporation of lithium ions into a dye–CD complex is
feasible for the formation of supramolecular gel and investigate
how the structure and property of the gel are influenced by the
lithium ions.
Supramolecular gels are formed from low-molecular-weight
gelators which manipulate noncovalent interactions to self-
assemble into a supramolecular network.1 These gels are held
together by multiple weak, and hence reversible, interactions.
Therefore, they are often considered as responsive and tunable
materials. Cyclodextrins (CDs), which include various
molecules in their cavity due to hydrophobic interactions
and/or hydrogen bondings to form inclusion complexes, have
proved effective in the preparation of supramolecular gels
which construct multi-dimensional nanoarchitectures from
CDs and their inclusion complexes.2 CD-based supramolecular
gels are constructed mostly from the secondary assembly of
pseudopolyrotaxanes that themselves are formed by threading
linear polymers of a high molecular weight into the inner
cavities of CDs. Such hydrogels have drawn an increasing
amount of attention due to their unique morphological
features and unprecedented properties; they are now widely
adopted in material and biomedical applications. Meanwhile,
the manipulation of metal–ligand bonding has been investigated
comprehensively in relation to the preparation of metallo-
gelators, in which both the ligand and metal ion play
complementary roles in the self-assembly of these materials.3
The advantage of this approach is to provide self-assembled
superstructures with the peculiar physicochemical properties
of metal ions.
First, we prepared a dark red solution made of a mixture of
dye 1 and g-CD at 60 mM. The anisotropic texture was visible,
and, when examined with a polarized optical microscope,
striations reflecting a diverse range of brightness could be
observed under crossed polarizers. We initially expected that
guest dye molecules would readily bind to metal ions,
since 8-hydroxyquinoline, a coupling component of dye 1, is
well-known as a ligand with high reactivity toward metals. On
adding lithium salt, however, the solution would not flow, thus
becoming a hydrogel (Fig. 1b). As the salt concentration
a Laboratory of Polymer and Electronic Materials,
Department of Nano Engineering & Textile Industry,
Dong-A University, Busan 604-714, Korea.
E-mail: jongpark@dau.ac.kr; Fax: +82 51 200 7540;
Tel: +82 51 200 7330
b Interdisciplinary School of Green Energy, Ulsan National Institute of
Science and Technology (UNIST), Ulsan 689-798, Korea
c Department of Material Science & Engineering,
Seoul National University, Seoul 151-742, Korea
d Department of Chemistry, Sangmyung University, Seoul 110-743,
Korea
Fig. 1 (a) Chemical structures of g-CD and dye 1, (b) photographic
images of the inclusion complexes ([dye 1] and [g-CD] = 60 mM) in
the presence of different LiCl concentrations (0, 0.6, 1.2, 30, 60,
e Department of Organic Material Science & Engineering,
Pusan National University, Busan 609-735, Korea
w Electronic supplementary information (ESI) available: Experimental
details about dye synthesis and gel characterization. See DOI: 10.1039/
c1cc10532a
360 mM, left–right), and (c) images from
a polarized optical
microscope of the complex in the absence (left) and presence (right)
of LiCl, respectively. The concentrations of dye 1, g-CD, and LiCl are
fixed at 60 mM. Scale bar is 200 mm.
c
4736 Chem. Commun., 2011, 47, 4736–4738
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increased, no apparent change was observed except for slight
spectral shifts. Gelation occurred only in the complex with
g-CD but was absent in those with a-CD and b-CD (Fig. S1,
supplemental data). The anisotropic structure remains after
gelation, with its texture embedded in the gel matrix (Fig. 1c).
When heated on a hotplate, the mixture becomes an isotropic
solution. However, when cooled to room temperature, it
returns to a gel state. Visual insight into the shape and size
of the hydrogel comes from electron microscopic measurements.
Numerous fibrils appeared to have formed a randomly
arranged network, according to SEM imagery. Meanwhile,
a TEM image showed bundles containing the thin, long
fibers that form the gel; the fibers have diameters of less than
100 nm and lengths of several micrometres (Fig. S2,
supplemental data).
Fig. 2 Schematic representation of the gel formation, by way of
inclusion complex, channel-type stacking, and tetragonal packing with
lithium ions.
stacking value of 21.8 g-CD (0.79 nm ꢁ 21.8 = 17.2 nm).
Tetragonal channel packing, more loosely packed compared
to hexagonal packing, allows the metal ions into the interstitial
sites between the g-CD stacks. Moreover, the stability of
the tetragonal packing in the presence of lithium ions is
strengthened by the formation of hydrogen attraction between
the metal ions and the neighboring host g-CD molecules. It is
also important to note that the presence of threaded dye
molecules inside the CD stacks facilitates the stabilization of
the channel structure. Therefore, we can reasonably deduce
the hydrogelation processes of the dye–CD inclusion complex
with the addition of LiCl (Fig. 2). In the model, fibrous rods
were formed by head-to-head g-CDs, held together by the dye
molecules in the cavities, and closely packed by lithium ions in
tetragonal interstices.
The 2D-ROESY NMR spectrum indicates the interaction of
the dye molecule inside the CD cavity in the presence of LiCl,
revealing the rotational NOE interactions between the inner
protons of g-CD and the phenyl protons of the dye (Fig. S3,
supplemental data). Measurement of the high-resolution magic-
angle spinning (HR-MAS) spectrum shows, after the addition of
LiCl, that the resonance signals of the aromatic protons decrease
significantly. Compared to those of other aromatic peaks, the
intensity of the peak around 8.9 ppm, attributed to –OH in
8-HQ, is relatively strong, which indicates that the dye is partially
covered by CD (Fig. S4, supplemental data). We observed no
changes in the intensity and shape of the FT-IR absorption
bands before and after adding LiCl, with u(CQO) and u(CQN)
occurring at 1570 and 1230 cmꢀ1, respectively, showing no
interaction of lithium ions with the threaded dye molecules
(Fig. S5, supplemental data). Upon the addition of Li+,
meanwhile, the proton signals of the CD were affected, as noted
by the perceptible upfield shifts in the 1H-NMR spectra (Fig. S6,
supplemental data). From these results, we conclude that lithium
salt locates outside the CD, forming preferential interactions with
CDs rather than the included dyes.
Rheological experiments are frequently employed in inves-
tigations of the origin and physical nature of the network.6
The viscoelastic information is significant from an application
standpoint. In Fig. 3a, the storage moduli (G0) and loss moduli
(G0 0) are shown as a function of the angular frequency (o). The
addition of LiCl leads to a noticeable increase in the strength
of the gel, as demonstrated by an increase in G0 (about 12% on
average), indicating that the interactions holding together the
anisotropic structure become stronger. All samples showed the
plateau regions of their dynamic moduli. The G0 values are
larger than G00 over the entire range of frequencies, showing a
substantial elastic characteristic. The strain amplitude sweeps
also reveal the elastic response of the hydrogel (Fig. 3b). The
G0 value rapidly begins to decrease above the critical strain
region (g 4 45%), followed by a collapse of the gel morphology.
While increasing temperature from 5 to 50 1C, the G0 values of
all samples decrease, whereas tan d gradually increases, during
which the interactions holding together the network structure
become weaker (Fig. 3c). Interestingly, the gel containing
The solution small-angle X-ray scattering (SAXS) data
obtained using radiation (l = 1.54 A) revealed a strong peak
at 2y = 7.51. This peak is attributable to the characteristic
reflection at the (200) plane of the tetragonal CD packing
(Fig. S7, supplemental data).5 While scanning at a lower
scattering vector, q, we observed several peaks at 0.039,
0.078, and 0.112 Aꢀ1, which suggest the presence of a layered
subunit with a regular spacing of 17.2 nm, corresponding to a
Fig. 3 Rheological data of the complexes ([dye 1] and [g-CD] = 60 mM). (a) Frequency sweeps (o = 0.01–100 rad sꢀ1) of three samples with
different LiCl concentrations (0, 1.2, 60 mM), (b) strain amplitude sweeps (g = 0.0625–625%) of the hydrogel containing LiCl (60 mM), and (c)
temperature ramp measurement (5–50 1C) of the same samples used in (a). The number in parentheses represents the concentration of LiCl.
c
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equimolar LiCl exhibits enhanced stability at an elevated
temperature compared to a pristine complex, showing no
crossover between G0 and G0 0. Such behavior indicates that
an increase in the ionic strength results in the formation of
hydrogels with higher storage moduli.
complex is concerned, lithium can be distinguished from other
metals, as it induces effective hydrogelation even in small
amounts (Fig. S8, supplemental data). In contrast, some metal
ions, such as Cu2+, Ni2+, Zn2+, Hg2+, Cd2+ and Pb2+
,
form precipitates, whereas the others show no change in their
appearance.
ꢀ
ꢀ
Other analogous lithium salts with CF3SO3 and ClO4
cause hydrogelation, while PF6ꢀ, due to its larger size and
weaker hydrogen bonding, leads to the denaturing of the
complex. Induced circular dichroism (ICD) spectrometry
provides a useful insight into their effects on the conformation
of the CD–dye hydrogel.7 The ICD spectra reveal negative and
positive Cotton effect peaks around 240 nm and 280 nm,
respectively (Fig. 4a). These two peaks are attributed to the
La and Lb transition band of the 8-HQs, respectively. We also
noted a negative Cotton effect peak around 380 nm coming
from the p–p* transition of the azo unit, which indicates that
the trans-azobenzene unit is encircled by a CD ring with a
direction parallel to the CD axis. The overall shapes of the
ICD spectra remain unchanged, but their relative intensities
vary slightly depending on the counter ions, suggesting that
the conformations of the inclusion complex are directly
affected by the counter ions.
In summary, this study presents an anisotropic supra-
molecular gel of CD–azo dye in which physical gelation is
completed by lithium salt. We have clearly demonstrated,
aided by various characterization techniques, that the
hydrogel consisted of fibrous rods formed by head-to-head
g-CDs including dye molecules in the cavities and that these
were held together by lithium ions. Rheological experiments
revealed the elastic characteristics of the hydrogel, and the
high ionic conductivities represented the good mobility of ions
inside the gel matrix. Hence, the observations in this work
can further be extended to subsequent research about the
rheological behaviors of the hydrogels containing different
lithium salts and the electrical properties of lithium-induced
CD–organogel, both of which are useful for lithium battery
applications.
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
funded by the Ministry of Education, Science and Technology
(Grant No. 2009-0070801).
Notes and references
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Fig. 4 (a) Normalized ICD spectra of (i) dye 1, (ii) the complex ([dye 1]
and [g-CD] = 60 mM), and the complex with (iii) LiCl (60 mM), (iv)
LiCF3SO3 (60 mM), (v) LiClO4 (60 mM), and (vi) LiPF6 (60 mM), and
(b) ionic conductivity of the complex as a function of the lithium salt
content.
3 F. Fages, Angew. Chem., Int. Ed., 2006, 45, 1680;
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The ionic conductivity (s) represents a significant parameter
in characterization of the ionic interactions, as it takes into
account the movement of any ionic species present in the
system. The lithium-induced anisotropic, supramolecular gel
shows s on the order of 10ꢀ3–10ꢀ2 S cmꢀ1 at room temperature
4 Y. Y. Li, J. A. Liu, G. Y. Du, H. Yan, H. Y. Wang, H. C. Zhang,
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(Fig. 4b). The s of the hydrogels (sgel) at a fixed salt concen-
tratiꢀon decreases in the order of counter ions Clꢀ 4 ClO4
,
ꢀ
PF6 4 CF3SO3ꢀ. This result closely correlates to a decrease
in the ionic mobility of the counter ions, which arises from the
larger atomic radii.8 Thus, counter ion mobility dominates the
conductivity in the hydrogel due to the immobilized lithium
cation inside the hydrogel matrix. Considering that solid
electrolytes provide conductivity that is several orders lower,
the current supramolecular hydrogel based on a noncovalent
approach9 deserves further study due to its relatively high
ionic conductivity.
9 H. Shimura, M. Yoshio, K. Hoshino, T. Mukai, H. Ohno and
T. Kato, J. Am. Chem. Soc., 2008, 130, 1759; T. Kato, T. Yasuda,
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There has been continued interest in the properties and
applications of lithium, and increasing pressure, therefore,
to introduce highly selective detection and separation
mechanisms for these applications.10 As far as the dye–CD
c
4738 Chem. Commun., 2011, 47, 4736–4738
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